1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave apparatus in which a surface acoustic wave element is sealed in a package.
2. Description of the Related Art
Conventionally, a surface acoustic wave apparatus is formed by connecting and fixing a surface acoustic wave element onto a base member through bump bonding, then by sealing the surface acoustic wave element in a package defined by bonding a cap member onto the base member. In the past, with such a surface acoustic wave apparatus, a seam welding method has been used and a Kovar alloy is used to define a joint so as to form an air-tight fixation between the base member and the cap member.
Further, sealing methods other than the seam welding method include a wax sealing method which requires that a wax material is applied to connecting portions of either the base member or the cap member, a small heater block is then contacted with for example the cap member, thereby heating the cap member so as to melt the wax material, followed by a cooling treatment to join the base member with the cap member.
When the above seam welding method is used to join the base member to the cap member, during a seam welding process, the connecting portion of either the cap member or the base member is only partially heated so as to be elevated to a high temperature, resulting in the base member and the cap member being joined together under a condition involving a significant temperature difference between the connecting portions and other portions. Then, with the cooling of the surface acoustic wave apparatus, a large residual stress is generated between the base member and the cap member, hence causing the base member 10 and the cap member 30 to deform significantly, as shown in FIG. 3. Subsequently, because of such a deformation, stress is concentrated on metal bumps 51 which have been used to connect and fix the surface acoustic wave element 20 on to the base member 10, and is also concentrated on electrodes 12 and 25 combined with metal bumps 51. Accordingly, there had been a problem that the metal bumps 51 and the electrodes 12, 25 would be damaged and are caused to peel off, resulting in some defective connections and defective characteristics.
When the wax sealing method is used to join the base member with the cap member, there also arises a problem that the base member would be deformed after the above joining treatment due the same reason mentioned above, causing problems similar to problems occurring in the seam welding method.
Note that it is necessary to provide a space for allowing for free vibration of the electrodes on the surface (surface acoustic wave propagating surface) of the surface acoustic wave element. However, since it is impossible to fill a space between the surface acoustic wave element and the base member with a resin using a process similar to forming a semiconductor device, it is impossible to reduce or eliminate the stress concentrated on the metal bumps, hence making it impossible to improve the joint strength. Accordingly, in order to improve the reliability of a surface acoustic wave apparatus, it is extremely important to reduce the stress exerted on the metal bumps.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of manufacturing a surface acoustic wave apparatus which reduces and minimizes a stress exerted on the metal bumps during a process of jointing the base member and the cap member, and prevents problems occurring in connecting portions of the metal bumps, thereby ensuring a sufficient reliability.
A preferred embodiment of the method of manufacturing a surface acoustic wave apparatus includes the steps of bump-bonding a surface acoustic wave element and a base member together through metal bumps having a melting point of about 450xc2x0 C. or higher such that the surface acoustic wave apparatus is fixed in a face down configuration to a bottom surface of a recess of the base member; and bonding a cap member and the base member with a wax material by heating the cap member and the base member uniformly at a temperature higher than a melting point of the wax material to melt the wax material.
With the use of the above manufacturing method, since there is no temperature difference between the different members during the process of joining together the base member and the cap member, a residual stress exerted on the surface acoustic wave apparatus after cooling treatment is minimized, thus greatly reducing the stress on the metal bump connecting portions. For this reason, it is possible to reliably and greatly reduce damage to the metal bumps and damage to the electrodes connected on the bump connecting portions, and to greatly reduce connection problems such as the peeling-off of the metal bumps or bump connecting portions. However, the surface acoustic wave element may be bump bonded or joined in advance by way of metal bumps having a melting point of about 450xc2x0 C. or higher, so as to be strongly supported and fixed on the base member while at the same time being electrically connected.
As the wax material, it is preferable to use a solder, an Auxe2x80x94Sn alloy, or a low melting point glass which has a softening point of not more than about 450xc2x0 C.
As described in the above, according to preferred embodiments of the present invention, the surface acoustic wave element is strongly connected and fixed on the base member via metal bumps having a melting point of about 450xc2x0 C. or higher and the surface acoustic wave apparatus as a whole is uniformly heated so as to join the base member with the cap member, thereby greatly reducing a stress exerted on the metal bump connecting portions. Therefore, it is possible to prevent the connection problems occurring in the metal bumps and the bump connecting portions, thereby obtaining a surface acoustic wave apparatus having a high reliability.